Influenza Infection, Newly Diagnosed Atrial Fibrillation, and One-Year Mortality in Hospitalized Adults: A Retrospective Cohort Study

Background/Objectives: Whether influenza increases the risk of newly diagnosed atrial fibrillation (NDAF) beyond the effect of acute illness is uncertain. We examined whether viral etiology and prior influenza vaccination (exposures) were associated with NDAF within one year (primary outcome), and whether NDAF was associated with one-year all-cause mortality (secondary outcome), in hospitalized adults. Methods: This retrospective cohort study included 21,653 adults hospitalized in Clalit Health Services facilities (2012–2020) who underwent respiratory-virus PCR testing: influenza A (n = 2829), influenza B (n = 992), other respiratory viruses (n = 561), and PCR-negative concurrent controls (n = 17,271). NDAF within 365 days was analyzed with cause-specific Cox and Fine–Gray models, including time windows; mortality with time-dependent Cox models; and vaccination with propensity score matching. Results: NDAF was recorded in 1228 patients (5.7%), 56% of them on the day of admission. Compared with PCR-negative patients, no viral etiology was associated with a higher hazard of NDAF (influenza A: HR 0.87, 95% CI 0.73–1.04; influenza B: HR 0.73, 95% CI 0.53–1.00), including within 30 days and after excluding day-0 events. NDAF was associated with higher one-year mortality (HR 1.68, 95% CI 1.52–1.87), especially when recorded after discharge (HR 3.04). Influenza vaccination was not associated with NDAF (HR 0.97, 95% CI 0.85–1.10). Conclusions: Among hospitalized adults tested for respiratory viruses, neither influenza nor prior vaccination was associated with NDAF compared with PCR-negative inpatients, who were not necessarily of similar acute illness severity. NDAF marked a higher hazard of one-year mortality.